irqchip: exynos: pass irq_base from platform

The platform code knows the IRQ base, while the irqchip driver
should really not. This is a littly hacky because we still
hardwire the IRQ base to 160 for the combiner in the DT case,
when we should really use -1. Removing that line will cause
a linear IRQ domain to be use, as we should.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Thomas Gleixner <tglx@linutronix.de>